Method for shaping a fiber optical device



Aug. 28, 1962 J. w. HICKS, JR.. ETAL 3,050,907

METHOD FOR SHAPiING A FIBER OPTICAL DEVICE w Filed June 27, 195e NVENTOES JOHN w. H/cks, Je. BY w/LFRED R AZ/Negd ATTORNEY 3,050,907 METHOD FOR SHAPING A FIBER OPTICAL DEVICE John W. Hicks, Jr., Fiskdale, and Wilfred P. Bazinet, Jr., Webster, Mass., assignors to American Optical Company, Southbridge, Mass., a voluntary association of Massachusetts Filed .lune 27, 1958, Ser. No. 745,092 4 Claims. (Cl. 49--84) This invention relates to` flexible fiber optical devices and has particular reference to an improved method of fabricating such devices.

Flexible fiberscopes which are formed of multiple strands of relatively straight glass bers, bundled together and joined in side-by-side relation with each other only at their opposite ends, are limited in their useful flexibility not only by the flexibility of the individual Ifibers but by the mutual interference of the fibers which takes place during the bending of such a fiberscope throughout its mid-section wherein the fibers are not attached to each other. That is, if a fiberscope of the above character is bent, the outermost fibers of the bundle must travel a longer path than the inner fibers. Since each Ifiber is practically inextensible the inner fibers of the tiberscopes must buckle or push through the outer fibers thereof with the result that a great deal of resistance to this buckling of the fibers and their pushing through the other fibers of the bundle is set up which acts to stiften the fiberscope and thus prevent it from being freely bent.

In general, it follows that the flexible section of a fiberscope of the above character is much less flexible as a whole than its fibers are when taken individually and for this reason, it has been necessary to construct such fiberscopes of very small fibers, which in themselves are extremely flexible, in order to attain a relatively high degree of flexibility in the composite structure of the fiberscope.

In order to avoid the difficult, costly and time-consuming task 'for yforming and handling extremely small lightconducting fibers in the manufacture of flexible fiberscopes, this invention provides an improved method of making fiberscopes which permits the use of larger lightconducting fibers with relatively little sacrifice of the overall flexibility of the bundle or assembly thereof as compared to the flexibility of the individual fibers.

A principal object of the present invention is to provide an improved method of making flexible fiberscopes.

Another object is to provide an improved method for making flexible fiberscopes of the character described above wherein the fibers throughout the exible midsection of the fiberscopes are so `formed as to be readily bendable in all directions as a unit without offering any substantial resistance to said bending.

Another object is to provide an improved method of making a fiberscope of the above character wherein the individual fibers throughout the mid-section thereof are so positionally arranged relative to each other as to render said mid-section highly flexible and readily bendable in all directions without introducing any appreciable buckling or other interaction between said fibers to resist said bending.

Another object is to provide a method of making flexible fiberscopes of the above character wherein their midsections are provided with an appreciable amount of twist throughout the length thereof whereby said twist to the mid-sections of the fiberscopes will permit the bending of the fibers therein as a unit in all directions without causing said fibers to buckle or push through each other thereby overcoming the resistance to bending.

Another object is to provide an improved Itechnique by 3,050,907 Patented ug. 23, 1962 which a bundle of relatively straight light-conducting fibers which are in closely fitted unattached relation with each other may be reformed by twisting and permanently set in twisted relation with each other while remaining unattached.

Cther objects and advantages of the invention will become apparent from the following description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a greatly enlarged longitudinal cross-sectional view of -a light-conducting fiber which is typical of the type used to form flexible ber optical devices;

FIG. 2 is an enlarged longitudinal cross-sectional view of a fiberscope assembly;

FIG. 3 is a diagrammatic longitudinal cross-sectional view of one form of apparatus which may be used to carry out the method of the present invention;

FIG. 4 is an enlarged side elevational view of a fiber optical assembly which has been formed in accordance with the invention;

FIG. 5 is a side elevational view of a fiberscope shown partially in cross-section and embodying a fiber optical assembly of the type illustrated in FIG. 4; and

FIG. 6 is a diagrammatic view of means for testing individual light-conducting fibers in accordance with the invention.

Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the various views, the invention is directed more particularly to the forming of flexible fiberscopes which are most generally constructed of a plurality of coated or clad light-conducting fibers 10 of the type illustrated by the greatly enlarged sectional view of FIG. 1. The fibers 10 preferably comprise an inner core 11 of high index int glass or the like having a relatively thin outer surrounding coating or cladding 12 of low index glass. Fibers of the above character may be formed by a variety of techniques such as extruding the high index glass along with the low index glass, placing a rod of high index glass within a close fitting tube of low index glass and drawing the assembly down to fiber size or alternatively drawing a fiber from a rod of high index glass and thereafter dipping or otherwise coating the ber with the low index cladding l2.

The term fiber as referred to herein is to be interpreted as including all light-conducting elements which are relatively long and small in cross-sectional area regardless of their cross-sectional configurations or degree of flexibility which is more or less dependent upon their dimentional characteristics and it should be understood that the particular fibers 10 have been given by way of illustration only and other types of fibers either coated or uncoated may be used if desired.

The forming of a flexible fiberscope assembly 13 such as illustrated in FIG. 2 is accomplished by placing a multii plicity of fibers 10 in grouped side-by-side parallel relation with each other and connecting the opposite ends of the fibers together with a suitable cement or the like 14 or by fusing the opposite ends of the fibers together. In this manner, the section of a fiberscope assembly between its opposite ends is free to ex since its fibers throughout said section `are in disconnected relation with each other. Reference may be had to patent application bearing Serial Number 703,914 filed December 19, 1957, by John W. Hicks, Jr. et al., patent application Serial Number 719,540 led March 6, 1958, by Henry B.Cole or patent application Serial Number 719,038 filed March 4, 1958, by Wilfred P. Bazinet, Ir., now abandoned, for more detailed information as to dierent specific techniques by which flexible fiberscope assemblies may be fabricated.

In all instances, fiberscope assemblies which are fofned of substantially straight light-conducting fibers are somewhat limited in their useful flexibility not only by the flexibility of the fibers themselves, taken individually, but by the mutual interference of said fibers which takes place when the mid-section of the tiberscopes are bent. The reasons for this limitation in flexibility have been discussed hereinabove and it is the purpose of this invention to' provide a process by which berscope assemblies of the above character may be rendered considerably more flexible and free to bend between their end parts.

Since the twisting of the conducting elements of cables or other similar articles will improve or increase the overall flexibility of the cable, it would obviously hold true that the twisting of the fibers throughout the midsection of a berscope assembly of the above character will render the fiberscope assembly considerably more flexible or free to bend.

When dealing with relatively fragile articles such as glass fibers 10, the problem of twisting a bundle of fibers of the character illustrated in FIG. 2 has heretofore been exceedingly precarious and difficult to perform. Furthermore, upon being twisted, the fibers must be locked or set in place to permanently hold said twist and must not become fused or otherwise attached to each other throughout the twisted area.

For purposes of best illustrating the method of the in- Vention, specific examples will be given of types of glasses from which the fibers 10 might be constructed and various temperatures to which the fibers must be heated during the reforming or twisting of the assembly thereof. However, it should be understood that for fibers which are formed of other types of glass or materials not referred to herein, different reforming temperatures would be used while still following the method steps of this invention.

If the overall flexibility of the berscope assembly 1l3 is to be increased only slightly to render said berscope adaptable to certain uses which require more flexibility than would be present in a structure such as shown in FIG. 2 but do not require a relatively high degree of flexibility, the fiberscope assembly 13 would be provided with only a slight amount of twist which may be applied while the fberscope 13 is at room temperature.

In FIG. 3 there is diagrammatically illustrated means for twisting the fiberscope assembly 13 and which embodies a stationary chuck 15 and a rotatable chuck 16. Each of the chucks 15 and 16 are provided with clamp screws 15a and 16a respectively between which the opposite ends 13a and 13b of the lberscope 13 are placed. Both of the chucks 15 and 16 are mounted internally of a heating chamber 17 upon the respective opposed side walls 18 and 19 thereof. The chuck 15 is bolted or otherwise xedly secured to the wall 18 as illustrated and the chuck 16, which is located in axially aligned relation with chuck 15, is provided with an integrally formed stub shaft 20 which is journaled in the wall 19 and extends outwardly thereof to be engaged by a drive mechanism which, for purposes of illustration, has been shown to embody a bevel gear 21 fixed to the shaft 20 and engaged by a second bevel gear 22 driven by a conventional electric motor or the like 23 through a gear reduction box 24 of any well-known design. While the chuck 15 has been shown as being secured to the side wall 18, it might alternatively be journaled in the side wall 18 and provided with drive means similar to that associated with the chuck 16 whereupon both chucks would be rotated in opposite directions to produce a twisting of the fiberscope assembly 13.

By clamping the opposite ends 13a and 13b of the fiberscope assembly 13 securely in the respective chucks 15 and 16 with the clamp screws 15a and 16a as illustrated and allowing a very slight amount of slack to the fibers extending between the chucks 15 and 16 so as to compensate for a slight shortening of the assembly 13 which will take place during the twisting operation, the mid-section of the berscope 13 may be twisted by operation of motor 23. Alternatively, the chuck 15 may be resiliently mounted on the side wall 13 of the heating chamber 17 with spring members or the like which are biased to draw the chuck 15 toward the wall 18 so as to provide a slight pulling force on the assembly 13 when clamped in the chucks 15 and 16. In this manner, the assembly 13 will be held fully extended at all times and any shortening thereof during a twisting operation Will be compensated for by the drawing of the chuck 15 toward the chuck 16 against the tension of its spring mounting. Furthermore, the tension on the assembly 13 will tend to prevent possible sagging thereof. It should also be understood that the apparatus of FIG. 3 may be mounted vertically on a bench or support rather than horizontally as shown.

In the above-mentioned case, where only a slight amount of twist is to be applied to the fiberscope 13, this is done at room temperature. However, since the fibers lill will inherently resist such twisting and tend to return to their initial shape when released from the chucks 15 and 16, it is necessary to lock or permanently set said fibers in place before removing the berscope assembly 13 from the chucks 15 and 16. This is accomplished by heating the fibers 10 of the fiberscope assembly to a temperature sufficient to produce a permanent set in the fibers without causing a fusing of said fibers or of the adjoining claddings 12 of the bers when clad fibers are used.

In order to obtain the proper temperature to which an assembly of fibers such as 13 must be heated to accomplish the above results, a simple determination may be performed (see FIG. 6) wherein a sample liber 34 of the same cross-sectional size and having identical core and cladding glasses as the fibers to be twisted according to the invention is tested to determine the temperature at which it will droop or sag. The determination is performed within a heating chamber such as 35 wherein a block 36 is provided to support the fiber 34. One end of the fiber 34 is placed on the block, as shown, and is held securely in place by a suitable weight such as another block 37. The fiber is allowed to extend horizontally away from the blocks 36 and 37 in cantilevered fashion a distance of approximately 500 times its diameter (for example, a .002 diameter fiber will be xed to extend a distance A of one inch from the blocks 36 and 37). Having thus arranged the blocks 36 and 37 and the ber *34, the temperature within the chamber 35 as gradually raised (for example, at a rate of Fahrenheit per hour). When the temperature within the chamber 35 reaches a predetermined point, the fiber will droop or sag, as shown by the dash line 34a. At the time the terminal end of the fiber droops a distance B of approximately one-quarter of an inch from its initial position, the temperature within the chamber 35 is recorded. The recorded temperature is the proper temperature to which the assembly 13 is to be heated to produce a permanent set in the fibers thereof without causing them to stick together.

It is pointed out that when using clad fibers to form the assembly 13, the deformation or melting points of the individual glasses themselves will not give a true indication of the proper temperatures required to -set the bers as discussed above. That is, the temperatures required to set certain clad fibers must be determined for the particular combination of glasses used to form the fibers in accordance with the size of the fibers and thicknesses of cladding. Therefore, by performing the abovedescribed determination, the proper temperatures for setting fibers having different combinations of high and low index glasses in an assembly such as 13 may be easily and accurately determined.

'By using the proper temperatures which are determined las outlined-above for setting the bers of an assembly 13, the fibers, in all instances, are held at this temperature for a time period sufficient to cause the heat to completely penetrate the assembly. For example, for an assembly 13 which is approximately 1/16 square inch in cross-sectional area, a time period of approximately one hour has been found to be sufficient to produce the desired results. It is pointed tout that regardless of the types of glasses used to form the fibers, the time period of one hour for assemblies of approximately the abovementioned cross-sectional area is adequate to set the fibers in their twisted relation with each other and by using the proper temperatures as indicated by the above initial determinations, no appreciable sagging or deformation of the assembly 13 as a whole will take place.

It has been found that an assembly 13 having fibers which are formed with a core part 11 of Shott F2 glass (which is a 1.62 index iiint) and a cladding of low index lead glass tubing designated as G12 by the Kimball Glass Co., both of said glasses being well-known in the glass industry and readily procurable, a temperature of approximately 910 Fahrenheit will produce lthe desired set without fusion. Heating coils 25 are provided within the chamber 17 and are suitably connected with conventional control means, not shown, to produce and maintain the desired temperatures within the chamber 17 during the heating cycle. At the end of the heating cycle, the assembly is exposed to the room atmosphere and allowed to cool at room temperature.

By twisting and heat-treating the fiberscope 13 in the above manner, a permanent set (see FIG. 4) will be produced in the fibers thereof and the fibers will remain in unattached relation with each other.

A fberscope thus reformed (see FIG. 4) will have a higher degree of flexibility than the same fiberscope when in the form shown in FIG. 2 since the twisted fibers 10 thereof will not have the tendency to buckle or push through each other when the fiberscope assembly is bent.

While a slight amount of twist will increase the flexibility of a fiberscope assembly, it should be understood that a greater amount of twist to the fibers 10 of the fiberscope assembly 13 will provide a higher degree of fiexibility t0 the structure.

In order to speed up the twisting operation without the danger of breaking the fibers, particularly in instances when a considerable amount of twist to the fibers of the iberscope assembly 13 is desired, it has been found that this danger of breakage may be greatly reduced by initially heating the fibers prior to twisting. In this instance, the assembly is preferably placed fully extended between the chucks 15 and 16 and is heated to a higher temperature than that stated for the determination given above. Unfortunately, however, since the fibers 10 are all practically equal in length and during a twisting of the assembly thereof the outermost fibers must travel a greater distance than the innermost fibers there is a tendency for the outer fibers to clamp down on the inner fibers at the higher temperatures and the claddings 12 of the adjacent parts of the fibers 10 might tend to fuse and thus cause a sticking together of said fibers. The proper temperature for twisting the assembly 13 while hot may be found by performing the abovedescribed determination and adding approximately 50 Fahrenheit to the above-mentioned recorded temperature. Thus, for fibers embodying the specific glasses mentioned above, a temperature of approximately 960 Fahrenheit should be used to perform a hot twisting operation of the assembly 13.

To aid in preventing a sticking-together of the fibers while twisting at temperatures of approximately 960J Fahrenheit, the fiberscope assembly 13 may be dipped in silicone oil or otherwise treated to provide a coating of silicone material around each of the fibers 10 throughout the area between their end parts.

With the fibers 10 of the berscope assembly 13 so treated with the silicone oil, the opposite ends 13a and 13b thereof are clamped Within the chucks 15 and 16 as described previously. The temperature in the chamber '17 is raised by the heating coils 25 to a tempera-ture of approximately 960 Fahrenheit and held at said temperature for a time period sufficient to allow the heat to thoroughly penetrate the assembly 13. For an assembly which is 1/{16 of a square inch in cross-sectional area, fifteen minutes is an adequate time period. The fiberscope assembly 13 is thereafter twisted very slowly at a rate of approximately -four turns per hour by operation of the motor 23. When a desired amount of twist has been applied to the assembly 13, the heating coils 25 are shut ofi, the motor 23 is stopped and the chamber 17 is exposed to the room atmosphere whereupon the fiberscope assembly will cool. When cooled to room temperature, the fiberscope assembly is removed from the chucks 15 and 16 whereupon its opposite ends, if not previously rendered receptive to light, are optically finished yand fitted with suitable optical elements or the like designed for the specific function `for which the fiberscope is to be used.

It should be understood that as in all glass flow or forming processes, other combinations of temperatures and time cycles of heating may be used to accomplish the desired twisting of the fibers and that the examples given are for purposes of illustration only of what has been found to be practical.

In FIG. 5 there is illustrated a twisted fiberscope assembly 13 which has been provided with mounting members 26 and 27 rigidly secured to its opposite ends and the members 26 and 27 are, inturn, secured in the adjacent ends 28 and 29 respectively of a flexible conduit 30 which has been formed of interlocking members 31 as illustrated. By `fabricating the members 26 and 27 and the conduit 30 of metal, the ends 28 and 29 of the conduit 30 may be soldered or otherwise securely fastened to the respective mounting members 26 and 27.

Prior to 4the attachment of the members 26 and 27 to the conduit 30, the fiberscope assembly 13 is untwisted slightly to loosen the strands or fibers thereof and thus render the fiberscope more freely flexible. By so loosening the fibers of the assembly, the effects of interfiber friction which might occur during bending if not loosened, will be obvated. Since the assembly 13 will have a tendency to return to its initial Afully twisted state, it is held slightly untwisted while the mounting members 26 and 27 are secured to the conduit 30 whereupon the conduit will thereafter permanently support the assembly 13 partially untwisted. For an assembly 13 which is approximately two `feet long and 1/10 square inch in cross-sectional area and initially twisted to the extent of one turn per each inch of length, a proper loosening of the twisted fibers thereof may be accomplished by rotating the opposed ends of the assembly in a direction of untwisting of the fibers by approximately one-half of a turn.

The mounting member 27 is provided with an objective lens 32 adjustably mounted thereon and the member 28 is provided with an eye lens 33` whereby image-forming light may be directed into the tiberscope assembly 13 by the objective lens 32, directed through the fibers of the assembly 13 and received yby the eye lens 33.

Other types of mountings may obviously be used to support a fiberscope assembly such as 13 in the manner just described or the assembly may be used in its fully twisted condition without the use of the conduit 30.

By twisting the bers 10 of the fiberscope assembly 13, as described above, the flexibility of the `structure as a whole is greatly increased as compared to a structure such as shown in FIG. 2 wherein the fibers 10 are not twisted. As mentioned previously, the twisting greatly reduces the mutual interference of the fibers which takes place in fiberscopes having substantially straight fibers. Furthermore, in constructing a fiberscope which is to have `a pre-established degree of iiexibility, it is possible to use fibers of larger cross-sectional size in twisted fiberscopes than would be the case in conventional straight fifberscopes. For example, a fiberscope made of four strands of .0l0 square fibers, if properly twisted, will have substantially the same overall fiexibility as a fiberscope made of 400 strands of .002 square fibers which are not twisted. Thus, it can be seen that by forming such fiberscopes in accordance with this invention a considerable saving in time and ex`- pense of manufacture may be had since the smaller bers are relatively diflicult to handle and more expensive to manufacture than the larger bers.

I-f a high degree of resolu-tion in a berscope is required, one may use rela-tively large multiple channeled bers which can be formed in the manner disclosed in John W. Hicks, Irfs patent application Serial No. 717,035, led February 24, 1958.

An alternative to the use of the silicone oil which, as described above, is used as an agent to prevent the bers 10 of the berscope assembly 13 from sticking together during the twisting process, is to replace the coating of silicone oil with .a coating of an organic material which when heated with a deficiency of oxygen will burn off and leave a ne grain carbon deposit on .the ber. Materials such as polyvinyl alcohol, polyvinyl b-utyrate or cellulose acetate in a CO2 atmosphere will provide a proper oxygen decient atmosphere yfor carbonation of said materials.

In place of the silicon oil the bers 10 of the berscope assembly 13 may be pre-etched with 30% hydrouosilicic acid which has been -saturated with silica. The pre-etching of the bers maybe accomplished by immersing the berscope assembly in the saturated uosilicic acid solution for la time period of approximately one hour and which is suflicient to produce a silica skeleton on Ithe outer surfaces of the bers. This silica skeleton which has a higher melting point than the glass of the bers will prevent a fusion of the adjacent surfaces of said bers when twisted at the approximate temperatures given hereinabove.

While only one specic example of glasses and reforming temperatures for the bers 10 has been given hereinabove, it should be understood that the method of the invention is readily `applicable to assemblies such as 13 which embody bers 10 yformed of other core and cladding glasses such as bers having core parts of a dense flint of 1.69 in index and `a cladding of sodalime tubing of 1.52 in index which would be twisted lat room temperatures and set by heating the same to a temperature of approximately 940 Fahrenheit for approximately one hour. If twisted while hot, an assembly of these bers would be heated to approximately 990 Fahrenheit.

A further example would be to form the bers 10 with core parts of the above-mentioned Schott 1.62 index `flint glass and claddings of Pyrex 1.48 index glass. In this instance, of 4twisted at room temperature, the bers would be set by heating the same to a temperature of approximately 1120 Fahrenheit :for a time period of approximately `one hour. If, however, the bers are twisted While hot, a temperature of approximately ll70 Fahrenheit would be used.

From the foregoing, it can be seen that simple, efficient and economical methods has been provided for accomplishing all the objects and advantages of the invention. Nevertheless, it is apparent that many changes in the steps of the method may be made without departing from the spirit of the invention as expressed in the accompanying claims and the invention is not limited to the exact matters shown and described as only the preferred matters have been given by way of illustration.

Having described our invention, We claim:

l. The method of making a exible assembly of a plurality of relatively long and thin light-conducting bers comprising assembling said bers in loosely grouped side-by-side longitudinally aligned relation with each other, securing only a relatively short section of said bers together at each end of said assembly, suspending said 4assembly from adjacent Ithe opposite secured ends thereof within a heating chamber, rotating at least one of said ends about a longitudinal axis passing `substantially centrally through each of the ends of said assembly an amount such as to impart a desired axial twist to said intermediate portion of said assembly, holding said assembly in said twisted condition and, at one stage of said suspending, heating -said assembly to a temperature approximating the temperature required to cause fa single ber of the same material and cross-sectional size as the bers of said assembly to sag approximately one-quarter inch when said ber is supported at one end so as to protrude horizontally to a free end thereof at a distance of approximately 500 times its diameter, shutting o`Ir said heat and allowing said assembly to cool to room temperature.

2. The method of making a exible assembly of a plurality of relatively long and thin light-conducting glass bers comprising assembling said bers in loosely grouped side-by-side longitudinally aligned relation with each other, securing only a relatively short section of said bers together at each end of said assembly, suspending said assembly from adjacent the opposite secured ends thereof within a heating chamber, rotating at least one of said ends about a longitudinal axis passing substantially centrally through each of the ends of said assembly an amount such as to impart a desired axial twist to said intermediate portion of said assembly, holding said assembly in said twisted condition, heating said assembly to a temperature `approximating the temperature required to cause a single ber of the same material and crosssectional size as the bers of said assembly to sag aprproximately one-quarter inch when said ber is supported at one end so as to protrude horizontally to a free end thereof at a distance of `approximately 500 times its diameter, shutting off said heat and allowing said assembly to cool to room temperature.

3. The method of making a exible assembly of a plurality of relatively long and thin light-conducting glass bers comprising assembling said bers in loosely grouped side-by-side aligned relation with each other, securing only a relatively short section of said bers together at each end of said assembly, suspending said assembly from adjacent the opposite secured ends thereof within a heating chamber, heating said assembly to a temperature a prroximating the temperature required to cause a single ber of the same material and cross-sectional size as the bers of said assembly to sag approximately one-quarter inch when said ber is supported at one end so as to protrude horizontally to a free end thereof at a distance of approximately 500 times its diameter, rotating at least one` of said ends about a longitudinal axis passing substantially centrally through each of the ends of said assembly an amount such as to impart a desired axial twist to said intermediate portion of said assembly, shutting olf said heat and holding said assembly in said twisted condition while allowing the same to cool to room temperature.

`4. The method of making a flexible assembly of a plurality of relatively long and thin glass light-conducting bers comprising assembling said bers in loosely grouped side-by-side aligned relation with each other, securing a relatively short section of said bers together at each end of said assembly, coating the surfaces of the bers throughout said intermediate portion with an agent to prevent possible sticking of the bers when subsequently heated, suspending said assembly from adjacent the opposite secured ends thereof within a heating chamber, heating said assembly to a temperature approximating the temperature required to cause a single ber of the same material and cross-sectional size as the bers of said assembly to sag approximately one-quarter inch when said bers is supported at one end so as to protrude horizontally to a free end thereof at a distance of approximately 500 times its diameter, rotating at least one of said secured ends of said bers about a longitudinal axis passing substantially centrally through each of the ends of said assembly an amount such as to impart a desired axial twist to said intermediate portion, shutting off the heat and holding said assembly in said twisted condition while allowing the same to cool to room temperature.

(References on following page) References Cited in the xle of this patent UNITED STATES PATENTS Hansell Mar. 25, Morgan Sept. 12, Lillie et al NOV. 18, Esser Dec. 23, Smison Feb. 23, Dodge etal. Mar. 14, Harris Jan. 22, Collins Oct. 7, Smison Oct, 4, Gwyn Oct. 25, Cook July 11, Rodman Oct. 10,

Waggoner Dec. 11, 1951 Stuetzer Sept. 2, 1952 Waugh Mar. 17, 1953 Searight Ian. 17, 1956 OBren Mar. 4, 1958 Sheldon Mar. 10, 1959 Grant Dec. 8, 1959 FOREIGN PATENTS Great Britain May 17, 1950 OTHER REFERENCES Handbook of Glass Manufacture, by Fay V. Tooley, 1953, page 41, Ogden Publishing Co., New York. 

